Corrosion: Causes, Effects, and Types

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Questions and Answers

What type of corrosion product formation leads to less corrosion of metals?

Porous corrosion products

Soluble corrosion products

Volatile corrosion products

Highly unstable corrosion products (correct)

How does humidity affect the rate of corrosion in metals?

Higher humidity increases corrosion rate (correct)

Higher humidity decreases corrosion rate

Humidity has no effect on corrosion rate

Humidity only affects corrosion at low temperatures

Which of the following factors also increases the rate of corrosion in metals?

Higher temperature (correct)

Low moisture content

Alkaline pH levels

Decreased acidity of the medium

Which pH condition is most detrimental to metal corrosion?

Acidic pH Signup and view all the answers

What is the principle behind cathodic protection?

Forcing the parent metal to behave like a cathode Signup and view all the answers

Which method of cathodic protection involves connecting to a more active metal?

Sacrificial anodic protection Signup and view all the answers

Which of the following metals is commonly used as a sacrificial anode?

Zinc Signup and view all the answers

Where is sacrificial anodic protection primarily utilized?

Buried pipelines and marine structures Signup and view all the answers

What is the first step in the process of electrodeposition?

Surface Preparation Signup and view all the answers

Which method is NOT typically used for cleaning a metal surface before electrodeposition?

Thermal treatment Signup and view all the answers

During the electrodeposition process, which component acts as the cathode?

The article to be metal-coated Signup and view all the answers

What is the primary role of the anode in electrodeposition using an inert material?

To continuously dissociate and replenish metal ions Signup and view all the answers

Which of the following factors is known to improve the quality of an electrodeposition deposit?

Maintaining optimum current density Signup and view all the answers

Which additive is specifically added to enhance the brightness of the electroplated deposit?

Thiourea Signup and view all the answers

What type of current is employed during the electrodeposition process?

Direct current Signup and view all the answers

How is the cleaned article prepared before it is connected in the electrolyte solution for nickel electroplating?

It is treated with an organic solvent and HCl Signup and view all the answers

What is the primary cause of corrosion in metals?

High energy state of pure metals. Signup and view all the answers

Which of the following describes dry corrosion?

Corrosion due to atmospheric gases in dry conditions. Signup and view all the answers

What happens to the metal's properties due to corrosion?

Loss of electrical conductivity. Signup and view all the answers

Which metal is least likely to undergo oxidation corrosion at high temperatures?

Gold (Au) Signup and view all the answers

What is a characteristic of stable metal oxides formed during corrosion?

They protect the underlying metal from further corrosion. Signup and view all the answers

What type of environment promotes oxidation corrosion?

Dry conditions with the presence of oxygen. Signup and view all the answers

What is one of the significant effects of corrosion on metal equipment?

It leads to contamination of products. Signup and view all the answers

Which of these metals is most resistant to oxidation corrosion at ordinary temperatures?

Silver (Ag) Signup and view all the answers

What type of oxide layer formed by noble metals like Au and Ag cannot resist oxidation corrosion?

Volatile oxide layer Signup and view all the answers

According to the Pilling Bed-Worth Rule, what determines if an oxide layer will be protective?

Volume of metal oxide is greater than the volume of metal Signup and view all the answers

Which gas is known to form a non-protective corrosion product when reacting with Sn?

Cl2 Signup and view all the answers

Liquid metal corrosion is most likely to occur in which scenario?

When a liquid metal flows over a solid metal at high temperature Signup and view all the answers

What is a characteristic of a porous oxide layer formed on metals?

Allows access of atmospheric oxygen to the underlying metal Signup and view all the answers

Which of the following metals is shown to have a volume ratio that leads to low corrosion rates?

Tungsten Signup and view all the answers

What essential condition is NOT required for wet or electrochemical corrosion?

High temperature environment Signup and view all the answers

Which of the following is NOT a gas that can cause corrosion in metals?

Nitrogen Signup and view all the answers

What occurs at the anode during the nickel plating process?

Ni -> Ni+2 + 2e- (Oxidation) Signup and view all the answers

Which component is NOT typically included in an electroless plating bath?

Electrolytic salts Signup and view all the answers

What is the main purpose of using a stabilizer in the electroless plating bath?

To prevent decomposition of the plating bath solution Signup and view all the answers

What is typically used to activate non-conducting surfaces before electroless plating?

SnCl2 and PdCl2 Signup and view all the answers

What role does formaldehyde play in the electroless copper plating process?

It serves as a reducing agent Signup and view all the answers

Which of the following is a component of the electroless plating bath that helps maintain pH?

Sodium acetate Signup and view all the answers

During which step are impurities removed from the surface of a base metal?

Surface pre-treatment Signup and view all the answers

What is the function of complexing agents in the electroless plating bath?

To stabilize the solution and avoid precipitation Signup and view all the answers

Study Notes

Corrosion

Corrosion happens when metals deteriorate due to chemical or electrochemical reactions with the environment.
Examples include rusting of iron and the formation of green film on copper.

Causes of Corrosion

Metals in their pure state are thermodynamically unstable and have a natural tendency to return to their more stable state.
This happens through reactions with the environment which form surface compounds.

Effects of Corrosion

Corrosion degrades the physical properties of metals like malleability, ductility, and electrical conductivity.
Reduces the efficiency of metallic structures.
Contaminates products if corroded equipment is used.
Requires time-consuming and costly repairs and replacements.

Types of Corrosion

Dry or Chemical Corrosion: occurs under dry conditions due to the direct reaction of gases like oxygen, chlorine, carbon dioxide, sulfur dioxide, nitrogen oxides, and anhydrous liquids with the metal.
- Oxidation Corrosion: Oxygen reacts with metals at ambient temperature in the absence of moisture.
  - Alkali and alkaline earth metals corrode at room temperature while high temperatures cause oxidation in most metals except for silver, gold, and platinum.
  - The nature of the oxide layer formed influences further corrosion:
    - Stable oxide layers protect the metal from further attack (e.g., oxides of lead, aluminum, and tin).
    - Unstable oxide layers decompose back into metal and oxygen, preventing oxidation corrosion (e.g., noble metals like gold, silver, and platinum).
    - Volatile oxide layers evaporate as soon as they are formed, exposing the underlying metal for further attack (e.g., molybdenum forms volatile MoO3).
    - Porous oxide layers allow atmospheric oxygen to reach the metal surface through pores, causing continuous corrosion (e.g., alkali and alkaline earth metals).
- Pilling-Bedworth Rule:
  - If the volume of the metal oxide formed is greater than or equal to the volume of the metal, the oxide layer is protective and non-porous.
  - If the volume of the metal oxide formed is less than the volume of the metal, the oxide layer is non-protective and porous.
- Corrosion by other gases: Gases like chlorine, sulfur dioxide, hydrogen sulfide, and nitrogen oxides react with metals to form protective or non-protective corrosion products.
  - Chlorine reacts with silver to form protective AgCl, but with tin, it forms non-protective SnCl4.
  - Hydrogen sulfide attacks steel at high temperatures in petroleum industries, forming porous FeS scale.
- Liquid Metal Corrosion: Occurs when a liquid metal flows over a solid metal at high temperatures (e.g., sodium metal corrodes cadmium rods in nuclear reactors).
Wet or Electrochemical Corrosion: Happens under wet or moist conditions, especially when a metal is in contact with a conducting liquid, or dissimilar metals are partially submerged in a corrosive environment.
- Requires the presence of anodic and cathodic areas, a conducting medium (electrolyte), and a potential difference between the anodic and cathodic areas.
- Factors affecting wet corrosion:
  - Nature of the metal:
    - If the corrosion product is unstable, corrosion does not occur.
    - If the corrosion product is porous or volatile, severe corrosion happens.
    - If the corrosion product is soluble in the medium, corrosion happens at a faster rate.
  - Nature of the environment:
    - Humidity: The higher the humidity, the faster the rate of corrosion.
    - Temperature: Higher temperatures increase the rate of corrosion.
    - pH: Metals corrode faster in acidic pH than alkaline or neutral pH. Amphoteric metals like aluminum and zinc corrode in basic mediums.

Corrosion Control Methods:

Cathodic Protection: Forces the metal to be protected to behave like a cathode, preventing corrosion.
- Sacrificial Anodic Protection:
  - Connects the metallic structure to be protected to a more active metal (sacrificial anode).
  - The more active metal corrodes slowly, protecting the structure by acting as an anode.
  - Common sacrificial anodes include magnesium, zinc, aluminum, and their alloys.
  - Used for buried pipelines, underground cables, marine structures, ship hulls, and metallic water tanks.
- Impressed Current Cathodic Protection: Uses an external power source to create a current that flow from the anode to the cathode (the metallic structure to be protected).
Protective Coatings: Apply a barrier layer to prevent direct contact between the metal and the environment.
- Metallic Coatings: Zinc plating (galvanizing), tin plating (tinning), chromium plating, nickel plating, etc.
- Organic Coatings: Paints, varnishes, lacquers, etc.
Inhibitors: Chemical substances that slow down the rate of corrosion by interfering with the corrosion process.
- Anodic Inhibitors: Form a protective film on the metal surface.
- Cathodic Inhibitors: Reduce the rate of the cathodic reaction.
- Mixed Inhibitors: Act on both anodic and cathodic reactions.
Alloying: Combine two or more metals to create an alloy with improved corrosion resistance.
Design considerations:
- Avoid sharp corners and crevices.
- Minimize the use of dissimilar metals.
- Use corrosion-resistant materials.

Electrodeposition

Electroplating: Depositing a desired metal on the surface of another metal using an electrolytic cell.
- Article to be plated is made the cathode, while the anode is either the metal to be deposited or an inert material.
- Metal ions migrate to the cathode and get deposited.
- Electroplating enhances corrosion resistance, wear resistance, and imparts decorative finishes.
Electroless Plating: Depositing a metal from its salt solution on a catalytically active surface using a chemical reducing agent without electricity.
- The process requires a pre-treatment step to activate the surface of the base metal.
- The plating bath contains the metal salt, a reducing agent, complexing agents, exaltants, stabilizers, and buffers.
- Electroless plating is used for various applications, including circuit boards, decorative surfaces, and functional coatings.

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Description

This quiz explores corrosion, a process where metals deteriorate due to chemical or electrochemical reactions. It covers the causes, effects on properties, and various types of corrosion, including dry and chemical corrosion. Test your understanding of how corrosion impacts metal structures and efficiency.